The present invention relates generally to a control circuit and more particularly, to a circuit for controlling the energy applied to a seal wire used in packaging machines for sealing plastic film.
Many packaging machines utilize a heated wire which is pressed against a pair of plastic film layers in a back-up pad to heat seal plastic wrappers. Typically, the film layers are sealed to one another and are cut by the heat and pressure of the wire. Too high a heat input into the film material can cause a number of problems such that the wire can have a build-up of melted film adhering to it, the back-up pad can burn out prematurely, and some films will smoke giving off noxious fumes forming a carbonaceous residue on the seal wire if the wire gets too hot. In the past, accurate temperature control has been difficult to obtain. One of the early techniques utilized to control the temperature of the sealing wire is seen in the Gassner U.S. Pat. No. 3,016,085, where the energy to the wire is controlled as a function of the linear speed of the web of film. Another approach is seen in the Virta, et al. U.S. Pat. No. 3,348,474, where the energy is applied during each machine cycle even when the sealing jaws are non-operative. Still further methods, as seen in the Rice, et al. U.S. Pat. No. 4,506,146, where the temperature control is based upon measurement of the changes in the resistance of the wire as it is heated. A further suggestion is seen in the Berg U.S. Pat. No. 4,377,738, where a power is applied until the seal wire element reaches a threshold temperature determined by the expanded length of the elements, and then repeated applications of voltage are made in response to the cooling and consequent contraction of the heating element as it cools below the threshold temperature. The Shanklin, et al. U.S. Pat. No. 5,321,230, anticipates some of the problems by reducing the time of the power impulse as the machine continues to run, and then returning the impulse to its long start-up time when the machine shuts down.
It often happens, however, that the packaging machine does not operate with the packages coming through at a constant rate. Often the line is abruptly stopped and then started again or sometimes, the packages are irregularly spaced so that the packaging machine will have to stop intermittently to wait for product. Essentially, the prior art systems run into errors in trying to properly control the temperature to meet the varying rates of production, the intermittent operation, and the cold start-ups that are common with the operation of packaging machinery. Each of the prior art patents has problems when in a packaging line environment. The Gassner U.S. Pat. No. 3,016,085 is designed to control sealing element temperature for longitudinal seals in film as it passes by the sealing wire. In the case of transverse seals, the film speed is meaningless because there is no movement of the film past the seal wire during the sealing process and the Gassner apparatus can not be used. The Virta U.S. Pat. No. 3,348,474 depends upon the machine cycling continuously to apply an impulse of energy for each machine cycle. The Virta apparatus will not work in a machine that merely waits (at rest) for the next product to arrive.
The Rice et al U.S. Pat. No. 4,506,146, would appear to be a theoretically correct approach to the problem since the temperature of the seal wire will always be maintained the same as measured by the resistance of the wire. In practice, however, it is a troublesome device. The resistance of the wire is very low and the change in resistance due to change in temperature of the wire is minuscule. Thus, any change in resistance of the entire sealing system, such as might be introduced by varying temperature in the transformer, corrosion in the connections, or minute changes in sensitivity or calibration of the sensing system, will interfere with the proper temperature control of the seal wire. The temperature control in the Berg U.S. Pat. No. 4,377,738, is dependent entirely upon maintaining a proper distance between the ends of the heated sealing element. Thus, as the sealing wire stretches due to repeated heating and cooling cycles, the set temperature of the sealing wire will change. Furthermore, expansion of the sealing arm on the machine as it heats up during operation can also change (increase) the operating temperature of the sealing wire.
The prior Shanklin et al. U.S. Pat. No. 5,321,230 overcomes some of the problems involved in other prior art. It is not dependent upon the resistance of the wire, nor is it dependent upon accurately measuring the length of the wire, and the duration of the heat applied to the sealing wire is shortened as the machine continues to run after a cold start. Some difficulty has been experienced, however, with intermittent product flow, since a short absence of product would return the time of voltage impulse to its highest setting. If the package rate was rather intermittent, the seal wire would be returned frequently to its highest setting. Therefore, long impulses of power would be repeatedly applied to the seal wire anytime there were significant gaps in product flow, thereby overheating the seal wire and seal jaw. This can cause melting or burning of the plastic film and can cause the jaw to be coated improperly with plastic residue from the film.